One Finnish reservist, who received the letter, said that the timing was by no means random.

He said: "It is clearly due to a more aggressive stance by the Russians.

"I’ve been in the reserves for 15 years and this is the first time I’ve received something like this. They send out letters like this very rarely.”

A senior research fellow at the Finnish Institute of International Affairs said they had been contacted by concerned individuals to ask if they should fear confrontation with Russia.

Charly Salonius-Pasternak said: “If Russia had headed down the path towards being a liberal democracy, there would not have been the pressure to do this.

It is clearly due to a more aggressive stance by the Russians

Finnish reservist

“In the current reality, it makes sense. The Finnish Defence Forces want to make sure that if they need to blow the whistle, they can rely on 230,000 reserves.”

She added: “That is linked to the increasing instability in the region. Russia has shown that it can transport large numbers of troops across vast distances very quickly.

"I have never had so many people coming up to me asking if they should be worried about the security situation.”

The Finland Army has 16,000 soldiers, but this number could swell to 285,000 if reserves were called upon.

The government has since denied that the letters are connected to tensions with Russia or the Ukraine crisis.

Instead, they said the plan for the mass delivery began two years ago, with the aim being to check they had correct contact details for all reservists.

Mika Kalliomaa, a spokesman for the Finnish Defence Forces: “The reservist letter is associated with our intention to develop communications with our reservists, and not the prevailing security situation.

Finland is widely assumed to hold a neutral status despite growing tensions between Russia and the West, in order to keep stable relations with its neighbour.

However, earlier this month Finland joined Britain to take part in the biggest Nato war games close to Moscow's doorstep.

The two-week exercise in the North Sea was aimed at improving anti-submarine warfare capabilities.